This invention relates to a method of producing foamed thermoplastic resin articles having smooth and glossy surfaces free from swirl marks and hair cracks.
In the conventional injection molding methods for foamed articles, resin containing a foaming agent is thermally plastified under a pressure which is capable of inhibiting the premature foaming thereof, and such thermally plastified, expandable resin is injected into a mold cavity. By way of such injection methods, Short Shot processes and Full Shot processes have been in use.
In the short shot process, the force of the expandable resin which is exerted in filling the mold cavity is so small that the expansion ratio of articles thus obtained is not high enough and that difficulty is involved in transferring the incuse design on the internal wall of the mold cavity on to the surface of the molded article in a faithful manner. Further, development of swirl marks is seen. Presence of such swirl marks detracts from the external appearance of the molded article and also makes the application of a coating difficult. To remove swirl marks additional finishing processes are required such as sanding, filling and polishing for which much labor is necessary. Furthermore, where the incuse design on the internal wall of the mold cavity is transferred as above, the aforesaid additional finishing processes are nearly impossible.
According to such U.S. Pat. as Nos. 3,211,605, 3,218,375 and 3,384,691, while the inner portion of resin which is full-shot into the mold cavity is a green and expandable soft state, a part of the said expandable resin is caused to be discharged out of the mold cavity by virtue of foaming and expansion so as to obtain an expanded article. In such methods, however, it is uneconomical to allow the plastified resin to discharge and another drawback inherent to such methods is that a high expansion ratio can not be hoped for. It is extremely difficult, what is more, to retain the weight of resin discharged each time at a constant value, with resultant substantialy fluctuation in the weight of molded articles obtained.
U.S. Pat. No. 3,058,161, Japanese patent publication No. 1964-22213 and others disclose a procedure whereby, after thermoplastified expandable resin is injected to fill into the mold cavity, within the period where the inner portion of the resin in the said mold cavity is still green and expandable, the aforesaid mold cavity is expanded to the predetermined capcity and shape so as to allow the resin to foam and expand. Because, in this method, the inner volume of the mold cavity is expanded forcedly by use of external force, the resin existing inside the mold cavity may, in so far as it possesses a foaming force being sufficient to follow and meet the aforesaid expansion of the inner volume of the mold cavity, be foamed, at an adequate expansion ratio, into articles including those with relatively thin secion thickness.
By means of the injection molding for foamed articles described in the foregoing, however, no fundamental solution has been attained to the trouble of swirl marks which is inherent to injection molded foamed articles. During time when the resin injected into the mold cavity flows inside the mold cavity while undergoing expansion, cells may be entrapped in the boundary surface between the wall of the mold cavity and the resin and the cell may be crushed and slipped by the flow of the resin, so that swirl marks may occur on the surfaces of molded articles as traces thereof.
Various methods have been devised aiming at prevention of swirl marks. U.S. Pat. No. 3,306,960 and No. 3,384,691, etc., for instance, disclose a method of heating the inner walls of the mold cavity to nearly about the softening point of the resin to be full-shot. However, such a method consisting of repeating the cycles of heating and cooling the mold with a large heat capacity entails consumption of thermal energy. This is not only very uneconomical, but longer cycle time is required for performing the molding as a matter of course due to the limitations of speed of heat transmission through the mold itself, thereby resulting in a fatal disadvantage of diminished productivity.
"Modern Plastics" a McGraw-Hill Publication, published in the United States on March, 1969, has presented the following method: In injecting thermoplastified expandable resin into the mold cavity, the mold cavity is filled in advance with a gas which is pressurized at a level registering in excess of the foaming pressure of the said injected resin, and then the thermoplastified resin is full-shot into the mold cavity. In such instance, the pressurized gas in the mold cavity is discharged through the relief valve, etc. by the injected resin having even higher a pressure. Then, while the inner portion of the resin is still green and expandable, the pressure inside the injection cylinder is reduced, and the inner portion of the resin in the aforesaid mold cavity is fed back into the above-mentioned injection cylinder for its expansion. Essentially, this is a method that has been improved over the aforesaid U.S. Pat. Nos. 3,211,605, 3,218,375 and 3,384,691; however, by means of those methods, as referred to above, increase in expansion ratio can not be hoped for, and wastage is inevitable because resin that has been plastified once is discharged out of the mold, into the injection cylinder, etc. Further, it is unavoidable that the density of molded articles will fluctuate greatly.
It is an object of the present invention to provide a method of producing foamed thermoplastic resin articles having smooth and glossy surfaces which are free from swirl marks and hair cracks.
Another objective of the present invention is to provide a method of economically producing the aforesaid articles enabling a faithful reproduction of an incuse design on the inner wall of the mold cavity on the molded articles as well as easy molding of articles including those with a relatively high expansion ratio.